Fluid Power - (ME353)- Lec12

8/17/2019 Fluid Power - (ME353)- Lec12

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Fluid Power Systems (ME353)

Fall 2012

Lecture 12


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Conditioning and Distributionof Compressed Air

Controlling Dirt, Moisture,Temperature, and Pressure


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Maximum pneumatic system operating efficiency is achieved when system

compressed air is:

– Consistently clean

– Free from moisture

– At a relatively uniform temperature

Pneumatic systems

need to carefully filterthe air taken into the

compressor intake to

extend the service life

of

–

Compressor – Other system

components


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The temperature of both the intake and compressed air is important

– Temperature changes are reflected in air pressure and volume per the

general gas law

– Temperature influences the ability of air to retain water vapor

Air in a pneumatic system may be cooled before, during, or after

compression

– Intake air temperature usually depends on the location of the compressor

air intake

– Intercoolers and aftercoolers are used to remove heat of compression

Intercoolers cool compressed air between the stages of a multiple-stage

compressor

Aftercoolers cool the air after the air has been compressed

Either air or water can be used as the cooling medium in these devices


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The receiver is the storage unit for compressed air

Typically, the receiver is a metal, cylindrical tank with domed ends

In addition to air storage, the receiver:

– Dampens system pressure pulsations

– Removes water vapor from system air

– In smaller systems, serves as the mount for the prime mover and

compressor

Formulas are available for calculating the volume needed for a receiver

These formulas consider:

– Cubic feet of free atmospheric air needed per minute

– Desired cycle time

– Atmospheric, initial receiver, and final receiver air pressures


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The air distribution system delivers high-pressure, conditioned air from the

receiver to workstations with a minimum of pressure drop

The type of distribution system depends on the size of the facility and the

level of demand for compressed air

Four general categories of air distribution systems are used with pneumatic

systems

– Centralized grid with fixed piping

– Decentralized grid with fixed piping

– Loop system with fixed piping

– Flexible hoses for portable compressor systems


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Centralized grid has one centralized compressor station and one line network

for a facility

Decentralized grid has individual compressors in several locations providing

air to smaller distribution networks


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Loop system has a main line that forms a continuous loop with compressors

located at one or more locations

This design provides maximum airflow with a minimum of pressure drop

between the compressors and the individual workstations


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© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.11

Air-Distribution System

Loop system


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Special attention must given to the setup of a hose air distribution in order to

minimize pressure drop

– Minimize hose length

– Reduce the number of couplings

– Eliminate kinks in the hose

Care must be taken to protect the hoses from abrasion in the work

environment

Typical hose distribution system


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Proper sizing of pipe for a fixed air distribution system is difficult because of

variety of work and loads conditions:

The pipe in air distribution lines should be installed with a pitch of 1 per

10 of line

– Allows liquid water to drain to water traps

– Water can be remove from traps either manually or with automatic

drain devices

Drop lines

lead from the main air distribution line to the workstations – Should be attached to the top side of the distribution line

– This prevents water in the distribution lines from entering the

workstation lines


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Final preparation of air at a workstation is accomplished by an FRL unit

– Air filter

– Pressure regulator

– Lubricator


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FRL air filter removes:

– Airborne dirt remaining in the atmospheric air compressed in the system

– Rust and scale from the interior of the distribution lines

– Liquid water that has condensed in the drop line

– Atomized oil from the operating compressor

Typical air filter uses

centrifugal force and porous

filter material to remove

unwanted materials from

system air

– Inlet passageway swirls

the incoming air,

creating a centrifugalforce that separates air

and contaminants

– Porous filter material

traps other undesirable

materials


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The pressure regulator in an FRL unit reduces system distribution line

pressure to the level needed by workstation tools and circuit actuators

Unit is also necessary as air pressure in the distribution line fluctuates due to

varying air demands and the characteristics of compressor-capacity control

Factors to consider when

selecting a system regulator are:

– Regulator style

– Pressure range

– Airflow range

– Conductor connection size

Several regulator designs are available for use in a pneumatic system

– Direct-operated regulator

– Basic, diaphragm-chamber regulator

–

Relieving-type regulator – Balanced-poppet valve regulator

– Pilot-operated regulator


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The lubricator in an FRL unit meters oil into pressurized system air at the

workstation

This provides lubrication for system valves, actuators, and air-powered tools

Rapidly moving system air passing

through a lubricator breaks up droplets

of oil, forming a mist or fog

This mist is transported through the

workstation lines to system components


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Effectively moving compressed air through a distribution system requires

appropriate conductors and connectors

Conductors can be classified as

- Rigid (Pipes) - Flexible (Hoses)

Conductors and the associated fittings must be properly sized and

selected to minimize the pressure drop (minimum length and minimum

no. of fittings and bends)


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Work Performers ofPneumatic Systems

Cylinders, Motors, and

Other Devices


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Pneumatic systems convert the potential energy of compressed air into force

and movement using:

– Cylinders

• Often called linear actuators

• Force generated is controlled by system pressure

• Speed of movement is determined by the volume of air allowed to enter the

unit

– Motors

• Often called rotary actuators

• Torque depends on air pressure and the internal structure of the motor

• Operating speed is determined by the internal displacement of the motor per

revolution and the volume of compressed air passing through the motor

– Variety of other specially designed actuators and processes to assist in or

complete a task

• Reciprocating movement

• Process assistance

• Nozzles

• Impact tools


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The basic structure of pneumatic cylinders is very similar to those used in

hydraulic systems

– Lower system operating pressures allow the use of lighter materials in

pneumatic-system components

– Water vapor present in compressed air requires the use of corrosion-

resistant materials or coatings for component parts


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Resilient seals prevent both internal and external leaks

Some manufacturers produce nonlubricated pneumatic cylinders

– Do not require the addition of oil to the system compressed air

– Special coatings on the surface of the cylinder bore and other bearing

surfaces provide lubrication – Coatings are not scraped off during the operation of the actuator


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Pneumatic cylinders may be single or double acting

Two factors are basic to determining required actuator size

– Cylinder force output

– Absolute air consumption required to produce desired system

performance The force output of a cylinder is determined by system air pressure and the

effective area of the cylinder piston F = P × A

Air consumption of a cylinder can be estimated

– Calculate the volume of air displaced during one cycle of the cylinder

– Multiplying the displacement volume by the number of cycles per

minute and the absolute pressure ratio CFM = V × Pr × N


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Pneumatic motors are used to power:

– Many large stationary machines

– A large variety of portable hand tools

These motors range in size from fractional-horsepower units to motors

producing over 50 horsepower


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Air motors are available in many designs

-Vane - Piston - Turbine -Other, specialized designs

The vane air motor is the most common design which can be found in hand

tools and large, stationary installations


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Piston air motors are most often found in installations requiring higher

horsepower output

They are available in both axial and radial piston designs


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Turbine motors incorporate air nozzles to direct air onto a turbine

– Nozzle produces high-speed air, which results in very high output-shaft

speeds

– In specialized applications, the speed of turbine motors can reach

100,000 rpm


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Pneumatic reciprocating motors use percussive or non-percussive techniques

to transfer energy from compressed air to a workpiece

– Percussive tools provide multiple, physical impacts to overcome

resistance – Non-percussive devices generally repeat a cycle to provide linear motion

that is used to directly operate a machine

Total input force of a percussion-type reciprocating motor is determined by:

- System air pressure - Area of the piston

The paving breaker is a very

common application of a

reciprocating motor

– Often called a jackhammer

– Used in a number ofapplications

– Models available ranging in

size from hand tools to large

units mounted on mobile

support equipment


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Reciprocating motors are used to power a variety of tools found in the

foundry, construction, and general metal fabrication industries

– Scaling hammers

–

Chipping hammers – Riveting hammers

– Tampers

– Rammers

Reciprocating motors commonly operate from under 1000 to over 3000movements per minute of operation

The operating rate depends on the task performed and the type of material

being cleaned, formed, or trimmed


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Compressed air is often used to assist functions without applying force to

linear or rotary actuators

– Spraying

– Drying

– Material agitation

– Material transfer

Compressed air is used to agitate material

– Assure proper mixing of liquids

– Prevent the settling of solid materials in a suspension – Provide oxygenation


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A nozzle is a very important device in many systems using compressed air

– In turbine motors, nozzles produce rapid airflow to assure the high speed

rotation of the turbine output shaft

–

Nozzles may be convergent or convergent-divergent

Blowguns are simple tools, but

must be very carefully handled

– Produce high air velocity

– Can cause serious injury

– Designs are available with

a bleed-off to increase

safety


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Pneumatic-powered impact wrenches are commonly used in service and

manufacturing industries

These tools are used to tighten or remove bolts and nuts


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Nail drivers are used to install staples, nails, or other fasteners in wood

and other materials

Pneumatic models have become popular for use by general consumers, as

well as by the construction industry

Pneumatic powered nail drivers use high-impact force from an air-driven

piston and piston rod to quickly and easily install fasteners


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Pneumatic rock drills are commonly used in quarries, mining, and road

construction

The holes bored by this equipment are used for the placement of explosive

charges that break the rock along a line connecting the holes


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Pneumatic-powered gripping tools are used by robotic equipment to handle

materials


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Controlling a PneumaticSystem

Pressure, Direction, and Flow


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The operation of a control valve depends on a series of control elements in

the valve

Control elements include:

– Fixed orifices

– Spools

– Pistons

– Diaphragms

– Precision-machined sliding plates

– Needle and poppet valves


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Varying the position of control elements within a control valve determines:

– Setting of the valve

– Accuracy of system control

The force used to position the internal elements of a control valve is provided by:

– Internal springs

– Internal air pressure

– External pilot pressure

– Electromagnets

– Manually applied force


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Pressure control is provided in three specific areas of a pneumatic system:

– Air distribution system

– Workstation areas

– Portions of circuits operating within the workstation areas

The pressure in the distribution lines of a pneumatic system is controlled by:

- Bypass - Compressor-capacity control system

Bypass pressure control limits maximum system pressure by bleeding excess

air to the atmosphere through a relief-type valve

Compressor-capacity control limits maximum pressure using one of several

different control methods

– Compressor start-stop

– Inlet valve unloading

– Compressor speed control


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Control of air pressure at the workstation is provided by a pressure regulator

It is often a part of the filter/pressure regulator/lubricator (FRL) unit

commonly used at the workstation

Pressure regulator is required at the workstation

due to:

– Air from the distribution line is at a higher

pressure than desired

– Varying distribution line pressure caused by

the compressor-capacity control system

When sections of a circuit require maximum air

pressures lower than that provided by the FRL unit,

additional pressure regulators may be inserted inthe circuit to provide the needed lower pressures


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Directional control valves can be grouped into four categories:

- Shut-off valves - Check valves

- Three-way valves - Four-way valves

Shut-off valves used in pneumatic systems include:

-Globe - Gate - Ball - Spool - Needle

These valves are used to allow or block airflow in circuit lines

Check valves automatically allow the free flow of air in one direction and

block airflow in the opposite direction

They are commonly used to block or allow air to flow around control valves

during different phases of circuit operation


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Three-way directional control valves:

– Pressurize a single-acting cylinder during extension

– Vent air during retraction to allow the cylinder to be returned to the

initial position by external forces


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Four-way directional control valves direct compressed air to power a

cylinder during both extension and retraction

They also power air motors during both directions of rotation

Four-way directional control valves are available in four-port and five-port

configurations

– A common exhaust is used in the four-port design

– The five-port design uses separate exhaust ports for the extension and

retraction positions of the valve


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In most directional control valves, resilient materials are used to seal the

space between the bore and the spool

– In packed-spool designs, sealing materials are attached to the valve spool

lands

– In packed-bore designs, sealing materials are attached to the bore in the

valve body


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Three center position configurations are commonly available for pneumatic

three-position, five-port valves

-Blocked center - Open center - Pressure center


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Flow Control Valves use tow common methods of controlling pressurized

airflow in pneumatic circuits which are:

– Fixed-size orifices

– Needle valves

Preferred method of flow control in pneumatic

circuits is the meter-out design

– Involves the placement of the flow control

valve in the outlet line of the actuator

– Favored due to the compressibility of air

Sh l l i ll l h hi h f


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Shuttle valves automatically select the higher pressure of two sources

Allows a circuit to always be connected to the highest pressure

Exhaust mufflers:

– Reduce the level of air exhaust

noise

– Prevent the entrance of dirt and

liquids into the exhaust ports of

directional control valves and

actuators

Fluid Power - (ME353)- Lec12

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